On the Stability of Direct Spring-Operated Pressure Relief Valves: Impact of Frozen Mixture Flow and Lift Restriction

Abstract

The current work explores the effect of (frozen) mixture flow and lift restriction on the stability of direct spring-operated pressure relief valves. First, we study the effect of frozen mixture (constant mass fraction) flow through a pressure relief valve with upstream piping. DIER's $\omega$ technique is employed to cope with the mixture parameters, notably sonic velocity and choked/non-choked flow through the nozzle. By means of one-dimensional simulation, we show that the change in sonic velocity plays a fundamental role in both the valve opening time and its stability. Due to the extremely low sonic velocities in certain range of water-air mass fraction, such valves will have a poor response time (slow opening) and chatter even for short inlet pipings. Next, we investigate the possibility of improving the dynamical performance of the safety valve by using a larger valve (i.e., a larger orifice) with a restricted lift option while keeping the vented mass flow rate through the valve constant. Our numerical investigations reveal that the additional restoring force emerging from the valve restriction at the upper stopper improves the stability behaviour in the case of gas applications but can hardly have any influence in the case of liquid service.